In motor vehicles a clutch is usually provided between a drive output shaft of the internal combustion engine and the drivetrain, in order to build up or interrupt the force flow from the internal combustion engine without jerkiness during the starting process. In a motor vehicle with a manual shift transmission, for shifting it is necessary to have very rapid and complete interruption of the force flow and the lowest possible mass moments of inertia. These requirements are fulfilled by a shiftable, dry friction clutch consisting of a diaphragm spring pressure plate connected to the driveshaft of the internal combustion engine, a clutch disk that can move axially on the input shaft of the transmission downstream and a disengagement device which, via a release bearing, transfers the release path from the non co-rotating actuation elements of the shifting clutch to the pressure plate. In the non-actuated condition of the shifting clutch the pressure plate is pressed by the diaphragm spring against the clutch plate, producing a frictional connection between the two plates so that both these elements rotate at the same speed. When the shifting clutch is actuated, the disengagement device acts upon the release bearing and presses the diaphragm spring in the direction of the pressure plate. This releases the pressure of the diaphragm spring from the pressure plate and results in disengaging the shifting clutch. Between the fully engaged and fully disengaged positions of the clutch the speed is controlled by the action of one side of the clutch on the other side of the clutch as a function of the pressure force of the diaphragm spring on the pressure plate and, correspondingly, that of the pressure plate against the clutch disk, so that the shifting clutch is acting as a speed converter. In such cases the torque is transmitted unchanged, but during speed conversion the torque transfer takes place with slipping, which leads to heating of the clutch and wear of its lining.
Unless excessive clutch wear is recognized in good time, the motor vehicle may be immobilized since transmission of the force flow from the combustion engine to the drivetrain is no longer possible. For that reason, to an increasing extent clutch wear indicators are provided, by means of which the wear condition of the shifting clutch can be monitored. This enables wear of the shifting clutch to be recognized at an early stage, so that the clutch can be replaced the next time that the motor vehicle is taken to a workshop.
From DE 20 2005 021 024 U1 a shifting clutch for the drivetrain of a motor vehicle is known, in which signal-emitting or signal-generating elements and a sensing element are provided, such that the wear condition of the shifting clutch can be determined from the separation of the signals received by the sensing element. For this purpose one of the signal-emitting elements is arranged on the pressure plate of the shifting clutch, while the respective other signal element is fixed on the housing of the clutch. When the drivetrain is in operation both signal elements sweep across the sensing element. The time separation of the signals then allows a conclusion to be drawn about the position of the pressure plate relative to the housing, and from this the wear can be estimated.
To those familiar with the subject other clutch wear recognition means are known, in which the clutch loading is calculated and the clutch lining wear is estimated on the basis of the loads. Furthermore, there are wear recognition methods that are based on clutch position, which determine the condition of the clutch by comparing the engaged position of the clutch when new, to the current engaged position.
The above methods of the prior art have the disadvantage that the clutch wear cannot be determined as an absolute value. For example, if the control unit is replaced the clutch wear values already determined are lost unless they have been transferred to the newly fitted control unit. Since the process of transferring clutch wear values already recorded into a new control unit can only be controlled with much difficulty in practice, during further operation of the motor vehicle concerned the wear calculations may be wrong and this can lead to replacement of the clutch for no reason, or to immobilization of the vehicle because the clutch has not been replaced in time.